patch 9.0.0206: redraw flags are not named specifically

Problem:    Redraw flags are not named specifically.
Solution:   Prefix "UPD_" to the flags, for UPDate_screen().
diff --git a/src/buffer.c b/src/buffer.c
index 669cfa9..1aaddd4 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1924,7 +1924,7 @@
     curbuf->b_last_used = vim_time();
 #endif
 
-    redraw_later(NOT_VALID);
+    redraw_later(UPD_NOT_VALID);
 }
 
 #if defined(FEAT_AUTOCHDIR) || defined(PROTO)